1. Field of the Invention
This invention relates to the field of materials-cleaning methods, and in particular it relates to methods for removing stains and deposits from substrates.
2. Description of the Art
Substrates of all types, including fabrics, finished surfaces, and fabricated metal, plastic, and ceramic surfaces, can, and often do, become stained or impaired with organic or inorganic pigments, oxidation products (corrosion), algae, mildew and/or other molds, salt accumulations, protenaceous matter, cellulosic matter, etc. A variety of physical and chemical procedures and compositions have been devised to dissolve or physically remove most if not all types of such organic and inorganic stains and deposits. For instance, it is known that acids, such as sulfuric, nitric, hydrochloric, and acetic acids and others, will dissolve a variety of inorganic stains and deposits such as metal salts, carbonates, and other materials and that they will chemically attack and weaken a variety of organic stains and deposits thereby facilitating the removal of such stains and deposits.
Sulfuric acid is one the stongest, readily available acids and, but for a number of significant disadvantages associated with use of sulfuric acid, it is an ideal candidate for the removal of surface stains and deposits. However, sulfuric acid is highly corrosive and therefore is difficult to handle, and it can cause sever corrosive damage to acid-sensitive mateials. It promotes a variety of side reactions with many mateials including dehydration, sulfonation, and oxidation, in which it is consumed, thereby eliminating its activity as an active chemical agent. Its heat of dilution in many solvents, particularly in water, is very high, and therefore the addition of sulfuric acid to water and/or other solvents must be handled with great care. These disadvantages are so severe that they eliminate sulfuric acid as a viable cleaning agent in many circumstances.
Jones, in U.S. Pat. No. 4,116,664, disclosed that the addition of urea to sulfuric acid to form certain combinations of urea and sulfuric acid resulted in a product which was non-toxic, non-caustic, and non-corrosive to black iron and that, therefore, it could be safely transported, handled, stored, and applied, as a fertilizer, with ordinary equipment. Both Jones, supra, and Verdegaal et al. (U.S. Pat. No. 4,310,343) disclose methods for producing certain combinations of urea and sulfuric acid. Verdegaal et al. disclosed methods of manufacturing urea-sulfuric acid compositions which contain at least 50 weight percent urea.
Other authors have also discussed the nature and manufacture of combinations of urea and sulfuric acid. For instance, D. F. du Toit, Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in chemical abstracts, 8, 2346, 1914) and L. H. Dalman, "Ternary Systems of Urea and Acid" JACS, 56, 549-53 (1934), disclosed that urea forms mono- and diurea adducts with sulfuric acid. Dalman also defined the phase relationships between the solid phase and saturated solutions containing urea and sulfuric acid at certain temperatures. While these authors described several characteristics of and methods for manufacturing combinations of urea and sulfuric acid, they did not observe that certain urea-sulfuric acid compositions are particularly active protenating and hydrolyzing agents in which the disadvantages customarily associated with sulfuric acid, such as its dehydrating, sulfonating, and oxidizing activity, are markedly reduced.